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Copyright 2010 Peterson Publications, Inc.

Plastics Decorating Magazine 
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Common Problems Associated with Pad Printing
by Liam Clancy

Decorating Workshop
July-August2004



In a perfect world, all pad printers would have their machine in a stable environment, one where room temperature, ink to thinner mixing ratios, machine cycle times, etc., are all controlled and kept consistent from one day to the next. Machine operators that are trained today would still be operating those same machines for years to come.

Unfortunately, with the exception of a few customers, this is rarely the case. Too often, the pad printing process is considered a poor relation to other more high profile processes, like injection molding or assembly operations, both of which may very likely be a company’s main source of income. Few customers appreciate the value that a printed logo, company name, or decoration brings to the final product.

The most common problems associated with the pad printing process are as follows:

1. Understanding the Process
Pad printing is not the ‘black art’ commonly referred to by those experiencing application problems. In order to maintain the process, you need to have a basic understanding of what actually takes place during the operation of any pad printing machine. Your machine or consumable supplier can explain this process best.

2. Consumable Life
It is recommended to always ask for guidelines on the expected life of your consumables. In most cases, the customer exceeds the expected lifespan in production. This is completely understandable in that cost is always a major factor. However, it should be understood that frequent changes of pads, clichés, and ink are common and more often than not, necessary, to keep a crisp, quality image.

3. Ink and Pad Selection
Quite often a customer will struggle to identify an ink suitable for a particular substrate, or a pad that will successfully transfer the image without compromising its quality. If you are in any doubt, your pad printing supplier is the best place to help you make these decisions. Often sample parts can be sent to your consumable supplier where they can be tested and the results reported back to you, usually at no charge.

4. Printing Un-treated Plastics
The wide use of polyethylene and polypropylene plastics, as well as their copolymers and alloys, has increased greatly in recent years. As a result, everyone using these plastics is searching for a printing ink that will not require the added cost and hassle of pre-treating.

Polyolefins are very difficult to bond to based on their non-polar, non-porous and chemically inert surfaces. Their high degree of chemical inertness to solvents generally limits the usefulness of solvent bonding as a viable printing technique. One successful approach to these materials involves proper surface pretreatment prior to bonding. The pretreatments, which are widely accepted, include chemical wiping, flame treatments, corona discharge and gas plasma.

On occasion, it is possible to find an ink that will stick to polyolefins without any need for pretreatment; unfortunately this is more the exception than the rule. This is likely to remain the case because many of today’s polyolefins consist of reground materials that offer no consistency from one batch to the next. The plastic properties remain but printing requirements drastically change.

Because so many factors are involved in the pad printing process, there may be as many as two or three reasons that you are not achieving a quality print. Having experience and knowledge of the process, as well as support from your supplier are the only guaranteed ways to make your pad printing operation successful. Hopefully the information and tables in this article will help with many of the more common challenges you might experience, but establishing good support from your machine and consumable suppliers is equally as important to your operation.

Problems and their Solutions

   
Ink not being picked up from the etch
Cause Solution
Ink is too thick This can occur at set-up or during the process itself. Solvent evaporation occurs throughout normal operation – add more thinner to the ink.
Ink is too thin Your ink may be drying out in the etch prior to the pad coming in contact with it. Increase the speed of your machine. Mix a new batch of ink.
Working with a new pad There may be a large amount of silicone on the surface of your pad. Wipe the excess oil from the pad using the same solvent used in your ink, remove any lint by applying low tack pad cleaning tape.
You may be using the wrong pad Speak to your supplier about correct selection.
   
Image not being released from your pad
Cause Solution
Ink may have dried on pad prior to application Speed up machine. Add retarder to the ink – this slows down the solvent evaporation rate.
Ink may be too wet Slow down your process. Thin out your ink.
Printing surface may be oily or un-clean Check your substrate for contaminants.
Pad may be too old Your pad needs a very thin layer of silicone oil on its surface to help release the ink. Over time your solvent based ink eats away at this silicone, the pad will appear dry. It is then time to replace your pad.
   
Image quality is poor
Cause Solution
All of the ink is not transferring to your substrate Ink may be drying to fast on pad.  Pad may be at the end of its life.
Insufficient downward pressure at ink pick-up Increase the vertical pressure on your pad at ink pick up.
Insufficient downward pressure at ink application Increase the vertical pressure on your machine in the printing position.
You may be using the wrong pad Speak to your supplier about correct selection.
   

Poor ink adhesion

Cause Solution
You are using the wrong ink Have your supplier suggest an alternative.
The print surface is oily or unclean Check your substrate for contaminants, clean if necessary with alcohol, wait for part to dry before printing.
Printing surface may require pre-treatment Get advice from your pad printing supplier on the most suitable method of treating.


Plastics Decorating would like to thank Liam Clancy, Managing Director of Teca-Print USA Corp. For further information he can be reached at (978) 667-8655 x 222 or at lclancy@tecaprint.com.